Which force pulls fluid out of the capillary?

The correct answer is capillary pressure, as this force plays a vital role in fluid dynamics at the capillary level. Capillary pressure is generated by the fluid inside the blood vessels and contributes to the movement of fluid out of the capillaries into the surrounding interstitial space. This process is essential for the delivery of nutrients and the removal of waste products at the cellular level.

The other forces mentioned also interact with fluid movement but serve different roles. Blood colloid pressure, for example, refers to the osmotic pressure exerted by proteins in the blood plasma, which tends to draw fluid back into the capillaries. Interstitial fluid pressure represents the pressure exerted by the fluid in the surrounding tissue spaces, which can oppose the movement of fluid out of the capillaries. Blood hydrostatic pressure, on the other hand, is the force exerted by the fluid on the walls of the blood vessels, which influences the push of fluid into the tissue but does not directly account for pulling fluid out.

In summary, capillary pressure is the specific force responsible for pulling fluid out of the capillaries into the interstitial space, making it the appropriate choice in this context.